The containers feature is enabled in Firefox Nightly 50 by default with the about:config pref privacy.userContext.enabled set to true. When enabled, containers will integrate seamlessly into your current browsing experience. You will have the option to open entirely new browsing contexts, which will have their browser storage (such as cookies or localStorage) separated from other containers. Your normal tabs, which we consider to exist in the default container, will still look and act as you'd expect them to before enabling containers.

Container tabs operate just as you would expect a normal tab to, except for the fact that the sites you visit will have access to a separate slice of the browser's storage. This means your site preferences, logged in sessions, and advertising tracking data won't carry over to the new container. Likewise, any browsing you do within the new container will not affect the preferences, logged in sessions, or tracking data of your other containers.